This invention relates to a transmission power control method on CDMA (Code Division Multiple Access) system, and apparatuses for embodying the method.
In the field of cellular telecommunications, recent techniques have been directed to CDMA system. In a CDMA system, a base station (BS) provides a service area or a cell-site (or a short cell-site) and communicates with multiple mobile stations (MS) of users existing on the cell-site wherein the communication from each mobile station to base station is called uplink or reverse link communication while the communication from base station to each mobile station is called downlink or forward link communication. In either the uplink and downlink communications, each mobile station uses a channel identified by a uniquely assigned digital code. Therefore, the reverse link and the forward link communications can be simultaneously carried out while sharing the same wideband frequency spectrums.
In a CDMA cellular system, transmission power levels of the mobile stations, especially in reverse link communication, become important. In reverse link communication, a base station simultaneously receives the signals from many different mobile stations at the same frequency. The CDMA demodulation processes taken into account, it is necessary that the signal received at the base station from each mobile station be as close as possible to single level. Conversely, the signal from one mobile station overwhelms the signal from another mobile station. This problem is well known as near-far problem.
To resolve the near-far problem, transmission power control of each mobile station is carried out in a CDMA cellular system so that the signal level received at the base station from each mobile station is as close as possible to a single predetermined level. Generally, the transmission power controls processes are categorized into an open loop power control process and a closed loop power control process. The former is suited only for median compensation in short period, while the latter can compensate instantaneous fluctuation of transmission power. The latter are specified, for example, in the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) IS-95B standard.
In IS-95B power control process, the base station transmits transmission power control bits for mobile stations in forward link communication. Each transmission power control bit indicates xe2x80x9cincreasexe2x80x9d or xe2x80x9cdecreasexe2x80x9d of the transmission power when having a value xe2x80x9c1xe2x80x9d or a value xe2x80x9c0.xe2x80x9d Specifically, a single transmission power control bit is transmitted every 1.25 ms, and step sizes of the power control are three, namely 1 dB, 0.5 dB, and 0.25 dB.
In IS-95B standard, as the traffic channels, a supplemental channel (SCH) may be used in addition to a fundamental channel (FCH). The fundamental channel is a basic traffic channel and always exists during the normal data transmission between the mobile station and the base station. On the other hand, the supplemental channel is added and used when the amount of the transmission data on communication between the mobile station and the base station becomes larger than a predetermined data amount which is allowed by the fundamental channel. Plural supplemental channels may be used in correspondence with the amount of the transmission data. It is noted here that the aforesaid transmission power control bits are used not for supplemental channel (SCH) but only for fundamental channel (FCH), in IS-95B standard.
In addition, packet communication compliant with IS-707-A can be build on an IS-96B CDMA system. In the packet communication, when the mobile station does not carry out data transmission/reception for a certain period, the mobile station is forced to be in dormancy, and the traffic channels for the dormant mobile station are opened or broken off. As the result of the traffic channels broken off, the aforesaid transmission power control bits are not transmitted to the mobile station that is in dormancy.
It is noted here that the mobile station can travel even in dormancy and the travel of the mobile station might cause the condition for the transmission power to change, However, the above-mentioned IS-707-A packet communication on an IS-95B system can not handle the change of the condition for the transmission power, because of no traffic channel during dormancy of the mobile station.
This invention therefore provides a transmission power control method which is carried out even if the mobile station is in dormancy.
According to one aspect of this invention, a transmission power control method comprises the following operations. Herein, the transmission power control method is carried out in CDMA system which comprises a base station providing a service area and a mobile station existing on the service area. In the CDMA system, the base station and the mobile station communicate with each other on the traffic channels of forward and reverse links during normal data transmission
In the transmission power control method according to one aspect of this invention, before the mobile station is in dormancy, power control channels of forward and reverse links are established between the mobile station and the base station. The power control channels of forward and reverse links are independent of the traffic channels of forward and reverse links, and are used only when the mobile station is in dormancy.
When the mobile station is in dormancy, a dormant power control signal is transmitted from the base station to the mobile station through the power control channel of the forward link. Then a dormant power control process is carried out at the mobile station, in accordance with the dormant power control signal transmitted from the base station, so that the mobile station produces a reference data signal which is subject to the dormant power control process. The power-controlled data signal is transmitted from the mobile station to the base station, so that the base station produces the dormant power control signal in accordance with a reception level of the reference data signal. Thus, closed loop power control for dormant mobile station is achieved.